Background: Single-agent PD-1 inhibitors have modest activity in the treatment of most STS. Potential strategies to increase efficacy include combination therapies targeting the tumor microenvironment. Considering that apart from direct growth inhibition and death of malignant cells, trabectedin (Tr) also induces macrophage depletion and/or different immunologic effects, suggesting a possible synergistic effect of combined Tr plus anti-PD-1 treatment. We therefore aimed to evaluate the efficacy and safety of combined Tr and nivolumab (Ni) as a second-line treatment in STS. Methods: The prospective, explorative, two group, non-randomized phase II NiTraSarc trial enrolled pretreated patients (pt) with advanced STS (Group A: lipo- or leiomyosarcomas, Group B: non-L-sarcomas). Pt were initially treated with 3 cycles of Tr 1.5 mg/m², followed by the combination of Tr 1.5 mg/m² + Ni 240 mg (“late combination cohort” (LCC)) for up to 16 cycles. After positive results of a preplanned interim analysis, pt received the combination therapy starting with cycle 2 (“early combination cohort” (ECC)). 92 pt were recruited to the trial (55 in Group A, 37 in Group B). Primary efficacy endpoint is progression-free survival rate after 6 months (PFSR6) according to RECIST v.1.1. This is a first analysis of the primary efficacy endpoint in Group B based on a modified intention-to-treat (mITT) population of evaluable 36 pt: 23 and 13 pt from the LCC and ECC, respectively. Results: The most common Group B subtypes comprised undifferentiated pleomorphic/not otherwise specified sarcoma (UPS/NOS, 13pt) and fibromyxoid sarcoma (FMS, 6pt). After a median follow-up of 5 months (m) PFSR6 was 13.9% for all pt, 8.7% in LCC and 23.1% in ECC. Median duration of disease stabilization (DoDS) was 4m in all pt, the LCC and the ECC. Two pt had a partial response (PR), 10 had disease stabilization (SD), while 13 pt progressed, and 11 had missing data. By subtype: PR- UPS/NOS=2 (DoDS 12.7m/12.5m). SD: UPS/NOS=3, epithelioid=2, synovial=2, FMS=1, fibrosarcoma=1, other=1. All 36 pt experienced at least one adverse event (AE) reaching a total of 579 AEs, 141 (24.4%) of which were considered to be grade ≥3 treatment-related AEs. The main grade ≥3 AEs were: leukopenia (47.2% of pt), neutropenia (41.7% of pt), thrombocytopenia (33.3% of pt), increased ALT (30.6% of pt), and anemia (27.8% of pt). Conclusions: Tr+Ni was well tolerated and showed activity in at least some patients with non-L-sarcomas (mostly UPS/NOS) especially in the ECC. Analyses of the collected data, including PD-L1 expression profile, with the goal to establish whether Tr+Ni should be further pursued in these patients, are ongoing. ClinicalTrials.gov Identifier: NCT03590210; EudraCT: 2017-001083-38. Clinical trial information: NCT03590210